A Direct Current-to-Direct Current (DC-DC) converter, or converting circuit, is commonly used in electronic devices. A typical DC-DC converting circuit has a BUCK circuit, or a BOOST circuit. FIG. 1 illustrates a BUCK circuit. When the driving signal is at a high level, the PMOS P1 is off, the NMOS N1 serving as a freewheeling transistor is on, the current on the inductor L1 is 0, the switch node (SW) is the drain of the NMOS N1, the change in the voltage of SW can reflect the change in the current on the inductor L1. In this case, the voltage of SW is 0, triggering a zero current detection circuit. FIG. 2 illustrates a BOOST circuit. When the driving signal is at a high level, the PMOS P2 is off, the NMOS N2 serving as a regulating transistor is on, the current on the inductor L2 is 0, SW is the drain of the NMOS N2, the change in the voltage of SW can reflect the change in the current on inductor L2. In this case, the voltage of SW equals the output voltage of the BOOST circuit, triggering the zero current detection circuit.
Typically, a zero current detection circuit is implemented with a comparator, as shown in FIG. 3. The positive output of the comparator C1 is connected to a reference voltage (Vref) and an intentional offset voltage (Voffset), the switch S21 is connected in parallel with Voffset. The negative input of the comparator C1 is connected to the capacitor CS21, switch S22, and SW. The negative input of the comparator C1 is connected to an output via the switch S24. The negative electrode of the capacitor CS21 is grounded via the switch S23. When the zero current detection circuit enters a sampling state, as shown in FIG. 4, the switches S21 and S22 are open, the switches S23 and S24 are closed, and the voltage of the capacitor CS21 is the sum of Vref and Voffset. When the zero current detection circuit enters a comparing state, as shown in FIG. 5, the switches S21 and S22 are closed, the switches S23 and S24 are open. When the sum of the voltages of SW and of capacitor CS21 is equal to Vref, the comparator C1 flips to output an enabling signal of zero current detection, in which case, the voltage of SW is equal to negative Voffset.
In a practical application, the zero current detection circuit has an operation delay (t-delay) and Voffset is set to be a constant. Thus, when the output voltage (Vout) of the DC-DC converting circuit increases, within the same time period of t-delay, the change in the voltage of SW is even greater. Although the voltage of SW for triggering the zero current detection circuit to start responding remains the same, after the t-delay, namely, when responding of the zero current detection circuit ends, the voltage of SW becomes even greater, indicating a larger current in the inductor, thereby reducing the accuracy of the zero current detection.
For example, FIG. 6 and FIG. 7 illustrate graphically the voltage of SW versus time in a zero current detection circuit for the BUCK circuit and for the BOOST circuit, respectively. For a BUCK structure, the output voltage Vout1 is greater than the output voltage Vout2, the slope of the curve of the voltage of SW corresponding to the output voltage Vout1 versus time is greater than the curve of the voltage of SW corresponding to the output voltage Vout2 versus time, Voffset is set to be a constant. Under the circumstance of the same t-delay, when the zero current detection circuit corresponding to output voltage Vout1 completes responding (namely, when the output voltage of the detecting circuit completes a logic flip), the voltage of SW is V1, time is T1. When the zero current detection circuit corresponding to output voltage Vout2 completes responding, the voltage of SW is V2, time is T2. It can be seen that, in the BUCK circuit, |V1−V2| is larger, that is, the voltage of SW at the completion of the response of the zero current detection circuit becomes even greater as the output voltage increases. In the BOOST circuit, |V1−Vout1| is much larger than |V2−Vout2|, that is, the difference between the voltage of SW at the completion of the response of the zero current detection circuit and the output voltage becomes even greater as the output voltage increases.